The Myc family of oncoproteins is widely considered to comprise critical regulators of cell proliferation, differentiation, and apoptosis. In addition, Myc oncoproteins are frequently deregulated and overexpressed in a wide range of cancers. Myc belongs to the larger class of basic-helix-loop-helix-zipper (bHLHZ) proteins and appears to function by forming heterodimers with the small bHLHZ protein Max. Myc-Max dimers recognize specific DNA sequences and have been shown to activate transcription at these binding sites proximal to promoter regions. Myc has also been shown to repress transcription of certain genes. While a number of putative Myc-Max targets have been identified, we still do not understand in detail how Myc exerts its complex effects on cell behavior. This proposal is concerned with three distinct aspects of Myc function. The first specific aim is focused on understanding the roles of two novel bHLHZ proteins, Mnt and Mga which, like Myc, form heterodimers with Max and recognize Myc-Max binding sites on DNA. Preliminary evidence indicates that these proteins have important roles in embryonic development. The PI proposes to examine the functions of these proteins using targeted gene deletions in mice and to define in greater detail their interactions with DNA and with other proteins. The second aim is to continue their initial studies using DNA microarrays to define the transcriptional program that accompanies activation of Myc throughout the cell cycle and in different cell contexts. They also propose to apply this technology to study the transcriptional role of the Myc antagonist Mad1 in mediating cell cycle exit during differentiation. The last specific aim is directed at gaining new information on Myc-Max functions by defining their interactions with other proteins. Using expression cloning and affinity purification schemes they have identified two new Myc/Max interacting proteins. They propose to further characterize these proteins and to identify other putative proteins associating with Myc-Max complexes.